(Related Art 1: Mechanical Fail-Safe)
As a hydraulic control unit equipped with an oil pressure control valve that generates a driving oil pressure corresponding to the displacement position of a pressure control valving element, there are know those equipped with a “fail-safe.” This fail-safe is used to avoid a problem that two hydraulically controlled objects are simultaneously supplied with driving oil pressure. (Refer to Patent Document 1, for example.)
Patent Document 1 discloses a technique to avoid a problem that multiple friction devices (examples of hydraulically controlled objects) are simultaneously supplied with driving oil pressure in the hydraulic control unit of an automatic transmission.
The automatic transmission includes multiple friction devices, such as a clutch and a brake. The automatic transmission appropriately actuates any friction device according to a vehicle running state (including the state of operation by an occupant) to attain a gear stage corresponding to the vehicle running state. Some automatic transmissions pose a problem when multiple friction devices are simultaneously engaged.
Here, friction devices that pose a problem when simultaneously engaged will be designated as first friction device CL1 and second friction device CL2; and oil pressure control valves that respectively generate driving oil pressure in the first and second friction devices CL1, CL2 will be designated as first oil pressure control valve CTR1 and second oil pressure control valve CTR2. Pilot valves for controlling the operation of the first and second oil pressure control valves CTR1, CTR2 will be designated as first pilot valve P1 and second pilot valve (not shown). (Refer to FIG. 30A)
The art disclosed in Patent Document 1 is so constructed that a fail-safe valve (changeover valve) J1 actuated by oil pressure is used to achieve “fail-safe (prevention of double engagement)” by mechanical operation.
Specifically, the art is implemented as illustrated in FIGS. 30A and 30B. That is, the fail-safe valve J1 is actuated by part of driving oil pressure supplied from the second oil pressure control valve CTR2 to the second friction device to block the oil passage running from the first pilot valve P1 to the first oil pressure control valve CTR1. (Refer to FIG. 30B) The actuation of the first oil pressure control valve CTR1 is thereby prohibited. (The first friction device CL1 is brought into a pressure exhaust state.)
The reference alphanumeral J2 found in FIG. 30A denotes a damper valve for absorbing fluctuation (so-called judder) in pilot oil pressure supplied from the first pilot valve P1 to the first oil pressure control valve CTR1.
(Problem Associated With Related Art 1)
The technique to use the fail-safe valve J1 to achieve fail-safe (prevention of double engagement) has an advantage that the technique can be implemented at a reduced cost because the cost of the fail-safe valve J1 is low.
However, this technique involves a problem. The engagement of the first friction device CL1 can be prohibited when the second friction device CL2 is engaged; however, the engagement of the second friction device CL2 cannot be prohibited when the first friction device CL1 is engaged. As mentioned above, the degree of freedom in designing an automatic transmission is reduced, and the low degree of freedom is detrimental in, for example, the recent trend to increase in the stages of automatic transmissions.
(Related Art 2: Electrical Fail-Safe)
Consequently, there have been proposed techniques to use respective oil pressure switches to electrically detect the engagement states of multiple friction devices to achieve “fail-safe.” (Refer to Patent Document 2, for example.) The “engagement state of a friction device” refers to a state in which torque transmission is carried out when the friction device is a clutch and a state in which torque braking is carried out when the friction device is a brake.
In the technique disclosed in Patent Document 2, a first oil pressure switch SW1 and a second oil pressure switch SW2 are provided as illustrated in FIG. 31. The first oil pressure switch SW1 is turned on when driving oil pressure supplied to the first friction device CL1 reaches a predetermined oil pressure (threshold value). The second oil pressure switch SW2 is turned on when oil pressure supplied to the second friction device CL2 reaches a predetermined oil pressure (threshold value). TCU (Transmission Control Unit: control device) 100 controls a second pilot valve and the second oil pressure control valve CTR2 so that the following is implemented: the second friction device CL2 is brought into a pressure exhaust state when the first oil pressure switch SW1 is turned on (when the first friction device CL1 is engaged). The TCU controls a first pilot valve P1 so that the first friction device CL1 is brought into a pressure exhaust state when the second oil pressure switch SW2 is turned on (when the second friction device CL2 is engaged).
The first and second oil pressure switches SW1, SW2 are switched according to driving oil pressure supplied to the first and second friction devices CL1, CL2. When a threshold value is set so that turn-on/off of the first and second oil pressure switches SW1, SW2 is switched in the course of shifting (in the course of reengagement), the following takes place: the range in which the first oil pressure switch SW1 is turned on and the range in which the second oil pressure switch SW2 is turned on are lapped over each other, which leads to erroneous determination.
To cope with this, threshold values for turn-on/off of the first and second oil pressure switches SW1, SW2 are set to a lightly higher value to prevent the following from taking place: the first and second oil pressure switches SW1, SW2 are simultaneously turned on in the course of normal shifting (in the course of reengagement). (This slightly higher value is a pressure value obtained in a state in which the first and second friction devices CL1, CL2 are engaged. This value will be hereafter referred to as “engagement detection threshold value.”)
For this reason, the first or second oil pressure switch SW1, SW2 does not serve any purpose other than carrying out the determination of fail-safe (determination of whether a friction device is engaged or disengaged).
(Related Art 3: Detection of Electrical Filling End Time)
To enhance the accuracy of automatic transmission shift control, there is demand for the detection of the filling end time of each friction device. The “filling of a friction device” refers to an operation to supply driving oil pressure to the hydraulic servo of a hydraulic actuator until immediately before a friction device starts to engage. (The time immediately before a friction device starts to engage is equivalent to a position where a piston of the hydraulic actuator starts to press against a frictional engagement portion, such as multiple discs.)
With the technique disclosed in Patent Document 2, however, the filling end time of the first or second friction device CL1, CL2 cannot be detected with the first or second oil pressure switch SW1, SW2. This is because the threshold values for the first and second oil pressure switches SW1, SW2 are set to an “engagement detection threshold value.”
To cope with this, the measure illustrated in FIGS. 32A to 32C can be taken. That is, the state of communication between an input port and an oil pressure switch SW is changed according to the displacement position of the pressure control valving element in an oil pressure control valve CTR. Then the displacement position of the oil pressure control valve CTR is detected from the on/off state of the oil pressure switch SW. Thus the filling end time of the friction device CL is detected by switching of the oil pressure switch SW.
More specific description will be given. As illustrated in FIG. 32B, the oil pressure control valve CTR is so provided that the communication between the input port and the oil pressure switch SW is blocked only when the friction device CL is filled. The oil pressure control valve CTR is so provided that the oil pressure switch SW is turned off only when the friction device CL is filled.
As illustrated in FIG. 32C, the oil pressure switch SW transitions from off to on when the displacement position of the pressure control valving element is as predetermined (filling end position), and the TCU 100 can thereby detect the filling end time of the friction device.
However, the oil pressure switch SW does not serve any purpose other than detecting the filling end time of the friction device CL.
(Related Art 4)
There is demand for the provision of the function of “electrical fail-safe” described in the above section of Related Art 2 and the function of “detection of electrical filling end time” described in the above section of Related Art 3.
To provide the two functions, it is required to separately provide an oil pressure switch SW for detecting the engagement state of a friction device CL and an oil pressure switch SW for detecting time when the friction device CL is filled.
The oil pressure switch SW requires connector cost, wiring cost, and the like in addition to the cost of the oil pressure switch SW itself.
The oil pressure switch SW for detecting the engagement state of the friction device CL does not serve any purpose other than the determination of fail-safe. The oil pressure switch SW for detecting the filling end time of the friction device CL does not serve any purpose other than detecting the filling end time of the friction device CL.
To install both the function of “electrical fail-safe” and the function of “detection of electrical filling end time,” a cost is increased and cost performance is significantly impaired.
(Summary of Related Arts)
To provide the following functions according to the related arts, one oil pressure switch SW is required for each function: “driving pressure detection” function for detecting the state of generation of driving oil pressure outputted from an output port (for example, the engagement state of a friction device CL); and “time detection (switching state detection)” function for detecting the displacement position of a pressure control valving element (for example, the filling end position in the displacement position of the pressure control valving element). As a result, a problem of significantly increased cost is incurred    [Patent Document 1] JP-A-9-303547    [Patent Document 2] JP-A-2001-116134